The present invention relates to an optical device for generating a rectangular pattern having a uniform light intensity distribution or profile, and more particularly to an optical pattern generating device which is suitable for application to an optical device for particle measurement or an optical recording device.
In the conventional optical device for generating a rectangular pattern from a laser such as He-Ne laser or Ar laser which provides an isotropic light intensity distribution having a Gaussian cross section, the rectangular pattern is usually generated by disposing a cylindrical lens having a uni-directional imaging property on the laser beam side of an imaging lens which converges a laser beam. In this method, however, since the laser beam has a light intensity distribution following a Gaussian distribution, an elongated elliptical pattern as shown in FIG. 1A is generated but a light intensity distribution of the pattern along its major-axis direction or in an x--x' cross section also exhibits a wide Gaussian distribution, as shown in FIG. 1B, which is constant in light intensity over only a narrow range and therefore cannot be regarded as being a uniform rectangular pattern.
For example, in a particle measuring device in which particles are brought into a rectangular area subjected to a focused rectangular light pattern to measure the shape of the particles or the number of the particles, there is encountered an inconvenience that if light intensities in the rectangular light pattern are not uniform, particles passing through a location where the light intensity varies cannot be detected as particles, thereby making it impossible to make a measurement with excellent accuracy.
Also, in an optical recording device using a laser beam, there is generated a recording pattern having an elongated elliptical profile, as shown in FIG. 1A, which is a desired rectangular pattern, that is, the recording pattern has not a satisfactory quality.
As mentioned above, in the case where the conventional rectangular pattern or elliptical pattern the light intensity of a peripheral portion of which is small as compared with that of a central portion thereof is used as illumination light for an object to be detected, there is an inconvenience that the accuracy of detection or measurement is undesirably different between the central and peripheral portions of the pattern.
The above type of techniques are disclosed by, for example, JP-A-51-19918, JP-A-51-84206 and JP-A-49-18304. But, the disclosed techniques only suggest an optical system using a single cylindrical lens or the combination of two cylindrical lenses and do not suggest the feature of the present invention.